Spawn rates are simply the rarity or probability of a Pokemon spawning in a particular spawner. We usually state the rarity of a Pokemon in the form of a chance e.g. 1/1000 - one in one-thousand chance, this means on average you would encounter a Pokemon of this rate 1 in 1000 spawns, this does not mean that after 1000 spawns it is guaranteed to spawn, this is because the rate resets and is recalculated after every new spawn. 

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The best way to imagine this is to imagine a die with as many sides as the rate of spawn, so to use the last example that would be a 1000-sided die, Let's say to spawn the 1/1000 you need to roll a 1, every time a new pokemon spawns you roll that die and if it lands on 1 the pokemon spawns, this also means you have a 999/1 chance to spawn something other than the 1/1000.

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Here is a very simple example of how spawners work for just the spawn rate:

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The in-game spawners have multiple layers of this to determine Mythical and Shiny then after this the Nature, Size, Gender and IVs are randomly assigned with their respective values and logic.

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Wiki Spawn rate representation:

on this wiki you will find some spawn rates written as 1/x<10 or 1/1 as opposed to the regular rates discussed above, these are exceptions made for ease of understanding, if you see a 1/x<10 this simply means that the rate of that Pokemon is less than 1/10, X representing the true spawn rate. 

Since 18/04/2025 the 1/x10 rate representation was removed. values are now shown to the closest whole value from 2-10.

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As for 1/1, this one means that it is the only Pokemon that can spawn in this spawner, this is reserved mainly for Legendary Pokemon.

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Myth busting:

it is a common misconception that spawners retain information of the previous or surrounding spawns, this is not true, each spawner calculates its own rates independently. previous spawns are not considered when a new spawn is calculated, spawners do not have quotas or limits on what they can output. theoretically, it's possible for a mew to spawn right after a mew has already spawned even though they are very rare.

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Staying at the same spawner for hours does not increase the odds of that spawner spawning any desired Pokemon, even if it does feel like it helps, it is just a placebo.

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